\usetikzlibrary{positioning}

\begin{tikzpicture}
  \node[draw, anchor=north] (B1) at (5,0) {$X=1$};
  \node[left=0.2em of B1.west] (B1text) {$\text{B}_{1}$};
  \node[draw, below=3em of B1.south, align=left] (B2) {
    (2) lb $a$   \enspace\enspace     (3) $t_{1}=x*5$\\
    (4) $r=t_{1}$ (5) $t_{2}=x<10$\\
    (6) if ($t_{2}$)\_
  };
  \node[above=0.1em of B2.north, xshift=2em] (B2text) {$\text{B}_{2}$};
  \node[draw, below=1.5em of B2.south, xshift=-4em, align=left] (B3) {
    (7) $t_{3}=x+1$ (8) $x=t_{3}$\\
    (9) gt $a$
  };
  \node[above=0.1em of B3.north west, xshift=0.6em] (B3text) {$\text{B}_{3}$};
  \node[draw, below=1.5em of B2.south, xshift=4em, align=left] (B4) {
    (10) ie \_\\
    (11) $r=0$
  };
  \node[above=0.1em of B4.north east, xshift=-0.6em] (B4text) {$\text{B}_{4}$};

  \draw[->] (B1.south) -- (B2.north);
  \draw[->] (B2.south) -- ([yshift=0.1em]B3.north);
  \draw[->] (B2.south) -- ([yshift=0.1em]B4.north);
  \draw[->] (B3.south) -- ([yshift=-0.8em]B3.south) -- 
    ([xshift=-0.8em, yshift=-0.8em]B3.south west) -- ([xshift=-4.6em, yshift=1.5em]B2.north west) --
    ([yshift=1.5em]B2.north);
\end{tikzpicture}
